Railway sleepers are an integral part of the “railroad track system” and are there to ensure the constant distance between the two rails, or more precisely, the distance between the inner sides of the mushrooms of the two rails forming the track and normally measured at the height of 14 mm below the rolling floor (so-called “gauge” in railway field) and to share the mechanical loads on the roadbed (or foundation bed, also known by the English term “ballast”).
It should be noted that the ballast is the part of the railway superstructure traditionally although not exclusively consisting of a cushion of crushed rock that supports the track drowned in it; moreover, the ballast ensures the geometric design conditions (level and alignment), absorbs the efforts exercised by the movement of the trains, gives elasticity to the track and creates a filter between track and environment in relation to vibrations.
As known, the railway sleeper made of wood has been for many years the most popular one in the market for railway installations but has created significant environmental damages in terms of deforestation and pollution of the subsoil by creosote.
Wood railway sleepers, indeed, were impregnated with creosote oil, a complex preservative chemical mixture which makes the wood resistant to attack by animal, plant pests and weather agents ensuring work durability of the sleepers themselves. Rail fastening systems used for wood railway sleepers have now become obsolete and expensive in terms of installation and maintenance as well as safety.
Furthermore, flexibility and lightness of wood railway sleepers have caused, and continue to cause in the few cases of use, problems related to pumping and dispersion of the ballast, thus increasing the need for burdensome and always unwanted maintenance interventions and related costs.
In addition, although they are impregnated with creosote oil, the average length of the life cycle of wood railway sleepers is not more than 15 years; this means high costs for replacing and disposing the sleepers nearly unusable because classified as special highly toxic waste.
Other problems related to the use of wood railway sleepers are related to the systems used for coupling of the rail, since wood degradation leads to non-compliance with the prefixed safety standards, especially regarding the tightness of the fastening systems of the rail.
At present wood railway sleepers are used only in sections of line with narrow curve radii, since they allow, during installation, adjustment of the gauge and, therefore, adjustment of the distance of the two rails.
The technology in the railway field has developed over the years for new types of sleepers, the most common of which is that one is made of prestressed reinforced concrete (also known by the acronym PRC).
Although they have an average life longer than wood, calculated from 20 to 25 years, PRC railway sleepers have shown some defects that involve equally high maintenance costs and safety issues.
Indeed, PRC railway sleepers are used in high-speed tracklines of the trains whose high loads subject the superstructure to heavy stresses; said loads, in combination with rigidity and weight of the sleepers themselves, as well as speed of the trains, cause pulverization of the ballast underneath which is reduced in sand becoming a soft and weak support system such that it does not ensure the proper and original altimetric and longitudinal features of the track.
Another obvious drawback related to the rigidity of the railway sleepers made of PRC is the structural weakening (caused by fractures, cracks and breaks) to which they are constantly subjected.
Railway sleepers of more modern and developed constructive conception, made at least partly of plastic material with high mechanical strength—hence the availability, on one hand, of railway sleepers integrally of plastic material and, from the other hand, of composite railway sleepers, composed of parts made of plastic material and parts made of concrete and/or metallic material such as typically steel), seek to remedy these drawbacks.
Such railway sleepers of known type, made of plastic material, present a completely homogeneous and uniform outer surface, without points of discontinuity.
However, while allowing to drastically limit the negative phenomenon of the pulverization of the ballast ensuring lengthening of the life cycle of the railway sleepers, they have found very little if null use in the high speed tracklines typical of European network, for which they result fully unsuitable and inadequate: this is due to their strong lightness which makes them suitable to be used in tracklines of a railway train not exceeding 80 km/h, typical for instance of U.S. and Chinese railway networks.
In addition, plastic material railway sleepers of the prior art cannot be used in the presence of elastic type fastening systems with direct laying of the rail on the sleeper and therefore they cannot be installed using mechanized installation or renewal of tracks systems and above all they involve complicated, toilsome, invasive, lengthy and expensive fitting works for mounting fastening elements that connect the rails with the sleepers themselves.
Rail fastening systems used on these plastic material railway sleepers are the same fastening systems with indirect laying used on wood sleepers, which are obsolete and highly expensive compared to the much safer and more economical elastic type fastening systems with direct laying of the rail matching, up to now, only the PRC sleepers.